The present invention relates to a method and apparatus for removing heat from electronic equipment, and in particular, a heat pipe system for removing heat from semiconductor chips and packages.
FIG. 1 shows a cross sectional view of a conventional semiconductor package 10. The package 10 includes a substrate 15, a lid 20, and a semiconductor chip 25. The semiconductor chip 25 is bonded to the substrate 15 utilizing solder and/or epoxy. Such a package 10 is often referred to as a xe2x80x98flip chipxe2x80x99 package, as the package is manufactured by xe2x80x98flippingxe2x80x99 the semiconductor chip 25 so that its terminals face terminals formed on a side of the substrate 15. Typically, ball-shaped solder terminals 30 are formed on either the terminals of the semiconductor chip 25 or the terminals of the substrate 15, or both. Thus, when the package 10 is heated, the solder balls 30 melt and create a reliable connection between the chip 25 and the substrate 15. Epoxy 35 may also be used in addition to the solder balls 30 to create a more reliable connection and provide stress relief.
When the package 10 is operated in its usual fashion, heat generated by the junctions of the semiconductor chip 25 is conducted through the chip and the lid 20, before exiting the package 10. Typically, heat is generated at the terminals of the semiconductor chip 25 and the terminals of the substrate 15, and therefore must pass through the solder 30 and epoxy 35, through the chip 25 body, and through the lid 20 before exiting the package 10.
In most cases the lid 20 is coupled to a heat sink or similar heat dissipation apparatus (not shown), to assist in moving heat away from the chip 25. The lid 20 is usually made of a low coefficient of thermal expansion (CTE) material such as Copper Tungsten (CuW) or Aluminum Silicon Carbonate (AlSiC). Such materials minimize the thermal stress caused by the mismatching of the CTE""s of the chip and the lid materials.
It has been shown that either AlSiC or CuW has a thermal conductivity large enough to effectively spread local, high heat fluxes. Previous attempts have been made to embed more conductive materials such as chemical vapor deposited (CVD) diamond and thermal pyrolytic graphite materials into AlSiC materials to achieve thermal conductivity values up to 1,000 Watts/m-K (meter-Kelvin). However, these approaches are generally quite expensive and cannot provide sufficient heat spreading performance at some very high heat flux conditions.
Heat pipes, and in particular flat heat pipes, have been shown to be able to spread very high heat fluxes (e.g., above 100 Watts/centimeter2 (W/cm2)) with minimal thermal resistances. In a typical application, a flat heat pipe has an equivalent thermal conductivity of at least 50,000 W/m-K, which is an improvement of approximately 50 times over the AlSiC-CVD diamond material. One example of a flat heat pipe currently being produced and used for this purpose is the Therma-Base(trademark) heat pipe produced by Thermacore, International, Inc. of Lancaster, Pa. (the assignee of the present application).
A basic heat pipe comprises a closed or sealed envelope or a chamber containing an isotropic liquid-transporting wick and a working fluid capable of having both a liquid phase and a vapor phase within a desired range of operating temperatures. When one portion of the chamber is exposed to relatively high temperature it functions as an evaporator section. The working fluid is vaporized in the evaporator section causing a slight pressure increase forcing the vapor to a relatively lower temperature section of the chamber defined as a condenser section. The vapor is condensed in the condenser section and returned through the liquid-transporting wick to the evaporator section by capillary pumping action.
Because it operates on the principle of phase changes rather than on the principles of conduction, a heat pipe is theoretically capable of transferring heat at a much higher rate than conventional heat conduction systems. Consequently, heat pipes have been utilized to cool various types of high heat-producing apparatus, such as electronic equipment (See, e.g., U.S. Pat. Nos. 5,884,693, 5,890,371, and 6,076,595).
However, conventional heat pipes cannot be bonded directly to most semiconductor chips due to the mismatching that occurs between the material from which the heat pipe is formed (e.g., Copper (Cu)), and the material from which the semiconductor chip is formed (e.g., Silicon (Si)).
Some have suggested that the solution may lie in conversion of the package lid itself into a heat pipe, thus avoiding the bonding problem. However, there are several shortcomings with this approach. First, AlSiC (i.e., the material from which the lid is formed) is chemically incompatible with water (one of the best working fluids for heat pipe cooling of electronics), and other possible fluids (e.g., refrigerants) cannot provide the necessary thermal performance without advanced and sometimes expensive wick designs. Second, Silicon (Si) and AlSiC are difficult to machine, thus increasing the manufacturing costs of such heat pipes. Finally, Tungsten (W) and Copper Tungsten (CuW) are heavy and expensive, and their compatibility with water is also questionable at best.
Therefore, there is currently a need for a system for effectively transferring maximum heat from a semiconductor chip package and having a CTE that is compatible with the chip package.
I.
The present invention is a semiconductor package including at least one semiconductor chip within a housing, the housing including a lid which overlies at least one semiconductor chip and a heat-dissipating device coupled to the housing, the heat-dissipating device including at least one area formed of a material with a low coefficient of thermal expansion.